Method and apparatus for correcting print images in an electrophotographic printer

ABSTRACT

A method of correcting print images in an electrophotographic printer wherein an image formed by a print head ( 3 ) on a peripheral surface of a photoconductor drum ( 1 ) is transferred and printed on a sheet of paper. The method comprises: furnishing the print head with a standard tacho signal in the form of pulses and thereby printing on a sheet of paper a check mark ( 9 ) for each given number of pulses of the standard tacho signal while the photoconductor drum makes one rotation; by means of a camera ( 10 ), detecting a periodic deviation in each spacing between successive such check marks; in response to the deviation in each spacing between the successive check marks, varying the pulse width of pulses of the standard tacho signal for each spacing between the successive check marks, thereby to provide a modified tacho signal; and effecting printing thereafter based on the modified tacho signal.

TECHNICAL FIELD

The present invention relates to a method and apparatus for correctingprint images in an electrophotographic printer using a photoconductordrum whereby a periodic deviation in image transfer position caused byworking distortion and/or fitting error for each of photoconductor drumsat their exchange time is automatically corrected.

BACKGROUND ART

In an electrophotographic printer, exchanging photoconductor drums in aservice or maintenance operation may give rise to an axial runout orout-of-round distortion due to working distortion or fitting error foreach photographic drum exchanged, which unevenly varies the distancebetween the peripheral surface of the photoconductor drum and its rotaryshaft center, that is its turning radius. Driving the photoconductordrum in such a state at a fixed angular speed of rotation causes theperipheral speed of the photoconductor drum to be varied or changedperiodically, whereby a toner image from the photoconductor drum onto asheet of paper is periodically deviated in transfer position. A changein peripheral speed of a photoconductor drum may also be caused by aradial runout or working error of a driving means such as gear, belt orthe like for driving the photoconductor drum.

To cope with such an inconvenience, the conventional means forcorrecting a deviation in transfer position of an image has been that inwhich for each formation of the image from a photoconductor drum aregistration mark is printed on a traveling body having a fixedregistration mark preprinted to detect an amount of deviation in spacingbetween the two registration marks at all times during each imageformation to effect correction of image forming position as shown in JP2,659,191 B.

The conventional technique presents the problem that a deviation must bemonitored at all times for each print image and that errors incorrection due to errors in reading the moving body for correaction ofthe deviation are entailed at all times.

In view of the aforementioned, it is an object of the present inventionto provide a method and apparatus for correcting printing images wherebya periodic deviation in image transfer position caused by workingdistortion and/or fitting error for each photoconductor drum can becorrected simply and less costly by only onetime measurement andcorrection at the time of exchanging photoconductor drums.

DISCLOSURE OF THE INVENTION

In order to achieve the object mentioned above, there is provided inaccordance with the present invention a method for correcting printimages in an electrophotographic printer wherein an image formed by aprint head on a peripheral surface of a photoconductor drum istransferred and printed on a sheet of paper, which comprises: furnishingthe print head with a standard tacho signal in the form of pulses andthereby printing on a sheet of paper a check mark for each given numberof pulses of the standard tacho signal while the photoconductor drummakes one rotation; by means of a camera, detecting a periodicdeviation, change or difference in each spacing between successive suchcheck marks; in response to the deviation in each spacing between thesuccessive check marks, varying the pulse width of pulses of thestandard tacho signal for each spacing between the check marks, therebyto provide a modified tacho signal; and effecting printing thereafterbased on the modified tacho signal.

And, in the print image correcting method mentioned above, after thecheck marks are made constant in spacing subsequent to furnishing of themodified tacho signal, the check marks are no longer necessarilyprinted.

The present invention also provides an apparatus for carrying out themethod mentioned above, i.e., an apparatus for correcting print imagesin an electrophotographic printer wherein an image formed by a printhead on a peripheral surface of a photoconductor drum is transferred andprinted on a sheet of paper, which comprises: a photoconductor rotaryposition detecting means for detecting a rotary position of thephotoconductor drum; a pulse generator means responsive to an inputsignal from the photoconductor rotary position detecting means forfurnishing the print head with a tacho signal in the form of pulseswhile the photoconductor drum makes one rotation; a control unit forcontrolling the tacho signal furnished from the pulse generator meansinto the print head to cause a check mark to be printed for each givennumber of pulses of the tacho signal on the sheet of paper while thephotoconductor drum makes one rotation; and a camera for detecting adeviation, change or difference in each spacing between successive suchcheck marks printed from the photoconductor drum on the sheet of paper,wherein said control unit is responsive to a detection value detected bythe camera to vary the pulse width of pulses of the tacho signal foreach spacing between the successive check marks for furnishing amodified tacho signal into the print head.

This method of correcting print images in an electrophotographic printerallows a periodic deviation in image transfer position on a sheet ofpaper caused by working distortion and/or fitting error for eachphotoconductor drum to be reduced by only onetime measurement andcorrection at the time of exchanging photoconductor drums.

Also, reading out check marks (registration marks) is only performed ina corrective operation and is no longer used in printing products.Hence, even in high-speed printing, no readout error may be produced anda deviation in image transfer position on a sheet of paper can beaccurately corrected, while making it possible to hold down the tonerand power consumption in mark printing and readout.

Besides, the aforesaid print image correcting method according to thepresent invention lightens the burden of an operator while exchangingphotoconductor drums, and improves the net working rate and operabilityof an electrophotographic printer.

The aforesaid print image correcting apparatus in an electrophotographicprinter according to the present invention allows the print imagecorrecting method mentioned above to be carried out less costly andsimply.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Drawings:

FIG. 1 is an explanatory view diagrammatically illustrating an apparatusfor carrying out a method according to the present invention;

FIG. 2 is an explanatory view illustrating a variation in peripheralspeed of a photoconductor drum for one rotation of the drum and amodification of the frequency of a tacho signal vis-a-vis such avariation;

FIG. 3 is a graph explanatory of modifications of the tacho signalfrequency vis-a-vis variations in peripheral speed of the photoconductordrum; and

FIG. 4 is a flow chart illustrating a method of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Mention is made of a form of implementation of the present inventionwith reference to the Drawing Figures.

FIG. 1 diagrammatically shows an apparatus for carrying out a method ofthe present invention. In the Figure there are shown a photoconductordrum 1, a backup roller 2 in rotational contact with the photoconductivedrum 1 and a print head 3 for forming a latent image on thephotoconductor drum 1. The latent image formed by the print head 3 onthe photoconductor drum 1 is visualized by a developing unit (not shown)which is provided opposed to a peripheral surface of the photoconductordrum 1 downstream of the print head 3 in the rotary direction of thedrum 1. The visualized image is printed on a sheet of paper (printablematerial) 4 traveling between the photoconductor drum 1 and the backuproller 2.

The print head 3 used here is an LED unit head, in which dot lightsources corresponding to print image dots are arranged in a line in alongitudinal direction of the print head to blink the dot light sourcesaccording to a print image. And, when a control unit 5 using a PC-BIP(Personal Computer—Bitmap Image Processer) is furnished with a tacho(TACH) signal and a CUE signal (printing start signal), the print head 3is furnished with printing data developed by the control unit 5.

The photoconductor drum 1 is provided on its one end face with an ironpiece 6, and at a position opposed to the end face of the photoconductordrum 1 provided with the iron piece 6 there is provided a proximitysensor 7 for sensing a passage of the iron piece 6 each for one rotationof the photoconductor drum 1. And, reading out a passage of the ironpiece 6 and thereby detecting an origin of the photoconductor drum 1 inits rotary direction, the proximity sensor 7 is arranged to furnish asignal of such detection to a pulse generator unit 8. From the pulsegenerator unit 8, a tacho signal of, e.g., 1260 pulses are furnished ina time from the origin to the origin, i.g., during one rotation of thephotoconductor drum 1.

The tacho signal is a timing signal in the form of pulses to cause theLED unit of the print head 3 to scan and emit light once every givennumber of pulses. The tacho signal in each cycle is composed of astandard waveform S1 of pulses of a fixed frequency, e.g., 1260 pulseswhich are produced from the pulse generator unit 8 in a time periodafter the origin is detected until it is next detected (during onerotation of the photoconductor drum 1).

Here, when e.g., at regular dimensional points Co1, Co2, Co3, . . . ,and Co126 every 10 pulses, scanning by and light emission from the LEDunit of the print head 3 is effected to expose the peripheral surface ofthe photoconductor drum 1 to light, successive checkmark images areformed thereon at a spacing C equivalent to 10 pulses and a check mark 9is printed on the sheet of paper 4. Then, 126 such successive checkmarks 9 are printed for one rotation of the photoconductor drum 1, overa peripheral length L of the photoconductor drum 1 (in FIG. 1 they areshown in a simplified view in this regard). And, the spacing C betweensuccessive check marks 9 is detected by a camera 10 and fed back to thecontrol unit 5.

However, due to working distortion or fitting error of thephotoconductor drum 1 or radial runout of the drive system, it has beenfound that there occurs a periodic variation or change in peripheralspeed of the photoconductor drum 1 as shown in the upper graph in FIG. 2while the photoconductor drum 1 makes one rotation. Then, with a fixedpulse width in standard waveform S1 of a tacho signal, the image (orspacing between adjacent check marks) becomes extended on a portionwhere the drum is higher (faster) in peripheral speed and the imagebecomes contracted on a portion where the drum is lower (slower) inperipheral speed. Consequently, the spacing C between adjacent checkmarks 9 printed on the sheet of paper 4 becomes varied with changes inperipheral speed of the photoconductor drum 1.

More specifically, the spacing C between check marks 9 on the portionwhere the image is extended is longer than the spacing by 10 pulsesbetween aforesaid successive regular dimensional points and for exampleis a spacing equivalent to 12 pulses or 11 pulses. On the other hand,the spacing C between check marks 9 on the portion where the image iscontracted is shorter than the spacing by 10 pulses between thesuccessive regular dimensional points and for example is a spacingequivalent to 9 pulses or 8 pulses.

In order to cancel out such extension and contraction of an image, it isproposed as shown in the lower graph in FIG. 2 to modify the tachosignal, i.e., to change the frequency of the tacho signal so as toincrease the frequency of the tacho signal (or to shorten the pulsewidth) at a portion where the image tend to be extended to effectprinting there in a shortened time and so as to decrease the frequencyof the tacho signal (or to lengthen the pulse width) at a portion wherethe image tends to be contracted to effect printing there in alengthened time.

Since the number of pulses of the tacho signal per one rotation of thephotoconductor drum 1 is fixed, increasing the frequency of the tachosignal in case the image tends to be extended means increasing thenumber of pulses per unit time then and decreasing the frequency of thetacho signal in case the image tends to be contracted means decreasingthe number of pulses per unit time then. Such modification of the tachosignal is performed by the PC-BIP in the control unit 5. And, by drivingthe print head 3 (causing it to scan and emit light) with a compensationvalue of the pulse width acquired by this control which is proper to theparticular photoconductor drum 1, it is possible to cancel the periodicextension and contraction of the image which has arisen from variationsin peripheral speed of the photoconductor drum 1 and to reducedeviations in image transfer position all the time while a product isbeing printed. Note here that once such compensation is completed, itbecomes unnecessary to print a check mark 9.

FIG. 3 is a graphical view illustrating the modification of a tachosignal vis-a-vis variations in peripheral speed of the photoconductordrum 1.

In FIG. 3, the number of pulses of a tacho signal per one rotation ofthe photoconductor drum 1 is 1260 pulses. The tacho signal is a timingsignal for causing the print head 3 to scan and emit light once per agiven number of pulses and has standard waveform S1 of pulses furnishedfrom the pulse generator unit 8 at a fixed frequency (with an equalpulse width) after the origin is detected until the origin is nextdetected (for one rotation of the photoconductor drum 1).

Here, the LED unit of the print head 3 is caused to scan and emit lightfor exposure on the photoconductor drum 1 at regular dimensional pointsCo1, Co2, Co3, . . . , and Co126 spaced apart from each other with afixed distance Co, e.g, of every 10 pulses of standard waveform S1.Then, checkmark images are formed on the photoconductor drum 1 at apitch equivalent to 10 pulses, namely at regular dimensional points(Co1, Co2, Co3, . . . , and Co126), respectively, whereby check marks 9are printed as shown in FIG. 1. And, these check marks 9 and theirrespective spacing or intervals are successively measured by the camera10 as at measurement points C1, C2, C3, . . . .

Thence, when the peripheral speed of the photoconductor drum 1 asmentioned above is periodically varied as V1, results of this aremeasured by the camera 10. As for spacing or intervals measured betweenmeasurement points C1, C2, C3, . . . , for the check marks 9 printed atthe pitch Co equivalent to 10 pulses, and their respective, immediatelypreceding measurement points, a spacing C1 d and a spacing C2 d atmeasurement points C1 and C2 where the peripheral speed is faster becomeequivalent to 12 pulses and 11 pulses, respectively, indicatingextensions of the image (or spacing between adjacent check marks). Onthe other hand, a spacing Cod, a spacing C5 d and a spacing C6 d as atmeasurement points C4, C5 and C6 where the peripheral speed is slowerbecome equivalent to 9 pulses, 8 pulses and 9 pulses, respectively,indicating contractions of the image.

The spacing C1 d, C2 d, C3 d, . . . , between measurement points C1, C2,C3, . . . , and their respective immediately preceding measurementpoints are detected by the camera 10 whose detection values are fed backto the control unit 5. The control unit 5 is arranged to output amodified tacho signal S2 as shown in FIG. 3, which is modified based onthose detection values.

The modified tacho signal S2 to cancel out extension and contraction ofthe image is to modify the standard tacho signal S1 so as to increasethe frequency such as at measurement points C1 and C2 where the imagetends to be extended to cause the LED unit of the print head 3 to scanand emit light there at an advanced timing and, conversely, so as todecrease the frequency such as at measurement points C3, C4 and C5 wherethe image tends to be contracted to cause the LED unit of the print head3 to scan and emit light there at a retarded timing.

To wit, the modified tacho signal S2 has the frequency increased untilit reaches regular dimensional points Co1 and Co2 so that between theregular dimensional point Co1 and its immediately preceding regulardimensional point there are delivered 12 pulses in a period of 10 pulsesin the standard tacho signal S1 and between the regular dimensionalpoint Co2 and its immediately preceding regular dimensional point thereare delivered 11 pulses in the period of 10 pulses in the standard tachosignal S1. Thereafter, the frequency is decreased so that between theregular dimensional point Co5 and its immediately preceding regulardimensional point there are delivered 8 pulses in the period of 10pulses in the standard tacho signal S1 and between the regulardimensional point Co6 and its immediately preceding regular dimensionalpoint there are delivered 9 pulses in the period of 10 pulses in thestandard tacho signal S1. In this way, the pulse width of pulses of thetacho signal is modified at each of regular dimensional points Co1 toCo126.

With a tacho signal modified as above S2, the LED unit of the print head3 is caused, once every 10 pulses to scan and emit light and to print acheck mark 9 as with the standard tacho signal S1. At measurement pointsC1 and C2 where the image tends to be extended with the peripheral speedof the photoconductor 1 made faster, pulses of the modified tacho signalhas a narrowed pulse width so that exposure with every 10 pulses iseffected at a timing quicker than with those before modification.Conversely, at measurement points C4, C5, C6 where the image tends to becontracted with the peripheral speed of the photoconductor 1 madeslower, pulses of the modified tacho signal has a widened pulse width sothat exposure with every 10 pulses is effected at a timing later thanwith those before modification. This allows checkmark images to beformed on the photoconductor drum 1 at a pitch Co that is an equivalentof 10 pulses, whereafter they are transferred to the sheet of paper 4.The check marks 9 then transferred onto the sheet of paper 4 are equallyspaced apart at a given spacing or distance C.

After it is confirmed that the check marks 9 printed on the sheet ofpaper 4 are equally spaced apart at the distance C, the print head 3continues to be furnished with the modified tacho signal S2, but checkmarks every 10 pulses are no longer printed.

A flow chart of the operations mentioned above can be described as shownin FIG. 4. As shown in the Figure, one first starts printing check marks9 on a sheet of paper 4 (step 1). Next, one counts a number of pulses ofa tacho signal detected by the camera 10 as between successive checkmarks 9 (step 2). Then, the counted number of pulses of the tacho signalbetween the check marks is compared with a preselected number of pulsesof the tacho signal in a fixed time interval between regular dimensionalpoints (step 3). If the counted number of pulses of the tacho signal isnot coincident with the selected number of pulses in the fixed timeinterval between regular dimensional points, pulses of the tacho signaloutput are altered into pulses of a modified tacho signal output (step4). Thereafter, one ends printing check marks 9 (step 5).

1. A method of correcting print images in an electrophotographic printerwherein an image formed by a print head on a peripheral surface of aphotoconductor drum is transferred and printed on a sheet of paper,characterized in that it comprises the steps of: furnishing the printhead with a standard tacho signal in the form of pulses and therebyprinting on a sheet of paper a check mark for each given number ofpulses of the standard tacho signal while the photoconductor drum makesone rotation; by means of a camera, detecting a periodic deviation ineach spacing between successive such check marks; in response to thedeviation in each spacing between the successive check marks, varyingthe pulse width of pulses of the standard tacho signal for each spacingbetween the successive check marks, thereby to provide a modified tachosignal; and effecting printing thereafter based on the modified tachosignal.
 2. A method of correcting print images in an electrophotographicprinter as set forth in claim 1, characterized in that after the checkmarks are made constant in spacing subsequent to furnishing of themodified tacho signal, the check marks are no longer printed.
 3. Anapparatus for correcting print images in an electrophotographic printerwherein an image formed by a print head on a peripheral surface of aphotoconductor drum is transferred and printed on a sheet of paper,characterized in that it comprises: a photoconductor rotary positiondetecting means for detecting a rotary position of the photoconductordrum; a pulse generator means responsive to an input signal from thephotoconductor rotary position detecting means for furnishing the printhead with a tacho signal in the form of pulses while the photoconductordrum makes one rotation; a control unit for controlling the tacho signalfurnished from the pulse generator means into the print head to cause acheck mark to be printed for each given number of pulses of the tachosignal on the sheet of paper while the photoconductor drum makes onerotation; and a camera for detecting a deviation in each spacing betweensuccessive such check marks printed from the photoconductor drum on thesheet of paper, wherein said control unit is responsive to a detectionvalue detected by the camera to vary the pulse width of pulses of thetacho signal for each spacing between the successive check marks forfurnishing a modified tacho signal into the print head.